The present invention relates to calorimetric flow meters, in particular calorimetric flow meters implemented as integrated circuits and to a method of manufacturing such a calorimetric flow meter.
Calorimetric flow meters measure mass flow and are used in industry and in the medical profession to measure the flow of a fluid. A typical calorimetric flow meter comprises a tube heated by a heater located halfway along the tube, the unheated ends of the tube remaining at ambient temperature. When the heater is on and no fluid flows in the tube the heating causes a temperature distribution in the tube whereby the temperature drops linearly with distance from the heater. When a fluid flow is present in the tube, heat exchange takes place between the tube and the fluid. Upstream from the heater fluid flow will cool the tube slightly and downstream the fluid flow will heat the tube slightly. The resultant temperature distribution along the tube is now uneven. Temperature sensors positioned halfway between the heater and the ends of the tube are operative to detect the difference ΔT between the measured upstream and downstream temperatures.
The temperature difference ΔT between upstream and downstream is proportional to the mass flow rate φm. The relation between the temperature difference ΔT and mass flow rate φm is also dependant on the heat conductivity of the tube and the heat capacity of the fluid.
The mass flow rate φm to temperature difference ΔT relation is not dependant upon the fluids viscosity or heat conductivity unlike for hot-wire or Venturi sensors advantageously making the calibration of a Calorimetric Flow Meter independent of the fluid. In order to make it convenient to use a Calorimetric Flow Meter in a wide range of industrial applications it is necessary to be able to produce a low cost robust Calorimetric Flow Meter. The best presently available Calorimetric Flow Meters are manufactured substantially from silicon and are potentially easy to damage.
It is therefore an object of the present invention to provide an improved Calorimetric Flow Meter.